There are known several industrial electrochemical processes making use of coated metal electrodes. A particularly relevant example is given by a chlor-alkali electrolysis process wherein cells equipped with nickel cathodes and titanium anodes are currently used. In order to decrease the energy consumption, which is a direct function of the cell voltage, a catalytic layer is applied on the nickel and titanium supporting substrates. A similar situation applies to other important electrochemical processes such as water electrolysis, metal electrowinning, electroplating and water treatment, among others. As most electrochemical reactions involve the evolution of gases, subjecting the relevant catalytic layer to a continuous mechanical stress, the adherence of such layer to the metal substrate plays a critical role in obtaining an industrially acceptable service life. It is known to those skilled in the art that the adherence of the catalytic layer is strictly related to the superficial roughness profile of the supporting substrate (hereinafter referred to as “accepting surface”), the roughness working primarily as an anchoring element.
The technical literature discloses several types of treatment for imparting roughness to an accepting surface. One procedure consists of sand or grit-blasting, wherein the metal surface is abraded by an impinging jet of either high pressure air with sand or metal grit, i.e., dry sandblasting, or of high pressure water with sand or metal grit, i.e., wet sandblasting. These treatments inject a substantial amount of energy into the metal structure, with consequent generation of internal stresses. When thin metal sheets are used, for instance of thickness below 1 mm, the internal stresses are likely to produce deformation with consequent loss of planarity. For this reason, dry or wet blasting can be applied only to relatively thick sheets. However, with thick sheets, the mechanical blasting action brings about a substantial hardness increase which may lead to cracks during application of the catalytic layer. A further drawback of this method is represented by the quality of the roughness profile, which is difficult to control and is dependent on a combination of several production parameters, such as size distribution of the sand or grit, pressure of air or water, size of the nozzles, and angle of the jet to the surface. In addition, the accepting surface, once the sand or grit-blasting is completed, is likely to be polluted by particles impinged on the metal which negatively affect the adherence of the catalytic layer. Finally, the sand or grit stock has to be discharged after a certain number of working hours as the particle size decreases with a consequently decreased efficiency of abrasion. The disposal of sand or grit which is polluted with the abraded particles of the treated metal substrates is difficult and expensive.
Other techniques known in the art for imparting roughness to an accepting surface include sand or grit blasting coupled with a first etching in HCl, heat treatment followed by etching, and melt spraying of metals or ceramic oxides allowing growth of a rough layer. Each of these techniques, however, have associated drawbacks.
Thus, it would be desirable to provide a method of pre-treating an accepting surface while avoiding the inconveniences of insufficient cleanliness of the pre-treated surface negatively affecting the adherence of the catalytic layer, the difficulty of predefining the extent of imparted roughness, the lack of reproducibility in the quality of the accepting surface both within the same sample and among the different samples of an industrial production, and the costs of disposal of the exhausted sand or grit stocks and of spent etching baths.